<template>
    <div class="box" :style="{ width: width + 'px' }"></div>
    <button @click="change">click</button>
</template>

<script setup>
import { ref } from 'vue'
let width = ref(100)
function change() {
    width.value += 100
}
</script>

<style>
.box{
  background:#d88986;
  height:100px;
  transition: width 1s linear;
}
</style>